Camera2: Many docs updates

- Improve CameraManager.AvailabilityListener docs
- Fix references to createCaptureSession immediately returning a session
- Remove most docs from deprecated methods, to minimize confusion.
- Remove references to deprecated methods from non-deprecated methods.
- Improve acronym use in metadata keys, and other minor metadata doc fixes.
- Improve package-level docs.

Bug: 15394187
Bug: 15394241
Bug: 15394242
Bug: 15394526

Change-Id: I48de626a33ef9b834578f7bb890de9b98d3122d2